Since it has been found that the administration of L-asparaginase to animals and man results in the selective destruction of the amino acid L-asparagine, which is specifically required for growth by certain tumor cells, the possibility that other enzymes may be useful in selectively lowering the concentration of some amino acids in vivo will be investigated. Purified enzymes such as L-amino acid oxidase, arginase, threonine deaminase, and others will be injected in mice. The effect of such treatments on the plasma concentration of amino acids, on the general health of the animals, and on the growth of tumors will be investigated.